Wind-powered electric power generation systems are aimed at converting kinetic wind energy into electrical energy and, in the case of grid-connected wind-powered systems, loading it therein for subsequent transport, distribution and use thereof.
In power grids with low penetration levels of wind-powered systems, the control systems that the operators of said grids apply to the rest of the generation units (conventional power stations) are generally sufficient to compensate fluctuations in the electric power injected by wind farms into the grid. These fluctuations are obviously produced by the variations inherent to wind resources. However, as the penetration of wind-powered systems in the grid increases, grid operators require said systems to participate in grid control operations, as in the case of conventional power stations.
The foregoing is equally valid for any renewable generation system that depends on a non-manageable resource, as in the case of photovoltaic systems and solar resources. Although we primarily make reference to wind-powered systems in the text of the invention, it must be understood at all times that it can be extended to renewable generation systems with non-manageable energy resources.
Maintenance of the balance between active, generated and consumed power in current power grids is carried out by maintaining system frequency at its nominal value (50 Hz in Europe, 60 Hz in the United States). When the power generated in the system exceeds that consumed, the system frequency rises with respect to its nominal value on accelerating the mechanical axles of the synchronous alternators in stations. On the contrary, when the power generated is less than that consumed, the frequency drops as the synchronous alternator axles slow down, reducing their speed. In order to compensate these deviations in frequency, in addition to those corresponding to active power, conventional stations are equipped with power controllers that respond to variations in frequency value in accordance with different system frequency control processes, such as primary, secondary and tertiary control.
Primary control allows restoration of the balance between active power generated and consumed by the grid. The combined operation of all the electricity generation units in an interconnected grid allows fast compensation of phase differences between consumed and generated power at any point of the grid. In most power grids, the legislation obliges conventional stations to establish a specific primary control capacity. This control consists of incorporating a control mechanism for proportionally increasing or reducing the power output reference value of the station and, in the opposite direction, to the variation in grid frequency, based on a characteristic referred to as statism. This characteristic is a straight line that descends on a coordinate plane where the horizontal axis is determined by the frequency variation with respect to its nominal value, as a percentage of this value, and the vertical axis by the variation in power with which the power station must respond at a given time to the corresponding frequency variations, said variation in power also being expressed as a percentage of the nominal power of the station. In this manner, statism is determined when the operator establishes the maximum frequency variation value to which stations must react, in addition to the maximum variation in power with which they must respond, with respect to the nominal power at a given time. Primary control must occur in small response times, in the order of seconds.
Secondary control allows restoration of power grid frequency to its nominal value. By means of said control, which is usually optional and remunerated, the system operator assigns new power generation values to the power stations, within control bands previously negotiated by the electricity companies that own the power stations. In this manner, power stations modify their power reference value until system frequency returns to its nominal value under the stationary regime. As opposed to primary control, secondary control occurs in response times in the order of minutes.
Finally, tertiary control, also remunerated, provides the power grid operator with more or less electricity generation capacity for the purpose of addressing possible deviations between predicted power consumption and expected electricity generation. In practice, tertiary control represents a change in the station programmed power set point, in such a manner that their operating horizon reaches values of nearly one or several hours.
At present, the constant increase in wind-powered electricity generation and that generated by other renewable sources with non-manageable resources, represents a significant challenge in power grid operation, the action protocols of which have developed over the years for a system based on manageable conventional energy sources. While assuming the inevitable variability of electricity consumption, statistical methods are currently capable of correctly predicting demand on a daily and hourly basis to a high degree. In this manner, operating protocols have allowed effective management of demand variability margin by conventional stations through the different control and operating services.
At present, the massive incorporation of renewable generation stations based on non-manageable resources (mainly wind farms) into power demand coverage brings additional uncertainty to grid operation, such as the unpredictable variability of said resources.
By way of example, we must point out the fact that in Spain, according to information provided by the company in charge of Spanish power grid operation, Red Eléctrica de España, in the early hours of the 30 of Dec. 2009, wind farm generation accounted for 54.1% of total generation, i.e. more than half of the electricity demand was covered by a non-manageable renewable resource. This degree of coverage represented a milestone in wind energy penetration and was successfully supported by grid operation thanks to the participation therein by the pumping stations and the reduction in the production of the thermal power stations to a technical minimum. Despite this, the low demand at the time obliged the operator to issue an order to cut back wind power generation by 600 MW for several hours. In similar situations produced in previous months, larger cutbacks were ordered, particularly in those cases where there was insufficient hydraulic pumping capacity.
The preceding example illustrates the fact that current operating protocols, even with the new grid operation-related technology (creation of renewable energy control centers, establishment of connections and communication with the generation control centers, installation of technical requirements for connection and communication, etc.), are reaching their limit in the integration of renewable energy sources, which will require power control services even in the case of electricity generation stations based on non-manageable renewable energy sources, including wind farms, in order to ensure grid stability as more renewable energy stations are incorporated thereto.
With regard to the primary control service and taking a wind farm as a representative example of a renewable energy station, different techniques have been proposed to provide this service using only the farm's wind turbines. In order for the wind turbines of a wind farm to provide the primary control service, they must operate at a maximum power value equal to the difference between the maximum wind power at a given time and the maximum power variation established by legislation for primary control (1.5% of the nominal power in Spain). This guarantees that, in the event that grid frequency drops to the minimum value established by the legislation and/or the operator's operating protocols, the wind turbine will have the necessary power capacity to raise it to the aforementioned maximum variation in power. The technical problem that this entails is that this process implies a constant loss (referred to as “discharge”) of wind energy, as the wind turbine operates under permanent regime almost always below the maximum extractable power in order to ensure that the power output variation margin fulfils the primary control.
There are patent documents that disclose hydrogen production systems powered by wind energy, as in the case of patents: WO2006097494, EP1596052, US20070216165, US20060125241 and DE10055973.
With regard to hydrogen production, there are basically two types of water electrolysis technologies: alkaline and polymeric membrane (PEM). The former are technologically developed and achieve very superior power values. An electrolyzer decomposes a water molecule to generate hydrogen and oxygen by applying electrical energy. The thermodynamic analysis of the system indicates the existence of a minimum supply of energy for this electrochemical reaction to take place in a sustained manner over time. In turn, the generation of hydrogen and oxygen in the electrolysis units must occur separately and be channeled inwards, avoiding the potentially explosive mixture of the two gases. At low production values, gas generation slows down, thereby increasing the risk of explosive mixtures. On the other hand, the purity of the gases produced depends, among other factors, on the operating point of the electrolysis system, worsening when said operating point is low.
In turn, current electrolyzers can be formed by one or several electrolysis units. In the event of including several units, operation thereof is always carried out jointly.
Due to the foregoing, current electrolyzers, whether formed by one or several electrolysis units, have a lower limit in their operating range below which the manufacturer does not allow operation thereof. This limit guarantees both safe operation of the electrolysis system and maintenance of the purity of the gas produced. Although the limit varies depending on the manufacturers, a representative range of current alkaline technologies could establish the limit at between 15% and 40% of the nominal power of the electrolysis system. This region within which the electrolysis system cannot operate represents a “dead band” (DB) for the system.
In order for the primary control service of a wind farm to be jointly carried out by the wind turbines and an electrolysis system, the size of the latter will be determined both by the primary control band imposed by the grid operator and its acceptable operating range, i.e. the power range above the lower operating limit of the electrolysis system or dead band. This requires considerable oversizing of the hydrogen system for the purpose of fulfilling the primary control service and avoiding wind energy losses, with the high economic cost that said oversizing entails. This is equally valid for any other type of control service that implies variations in the power injected into the grid.